Conventional methods used to study fine details of magnetic materials include the Bitter Method (as first suggested by Bitter, F. in Phys. Rev. 38, 1903 (1931) and Von Hamos, L. and Thiessen, P. A., Z. Physik 77, 442 (1931)) and Lorentz microscopy (Cullity, B. D., "Introduction to Magnetic Materials"; Addison-Wesley Publishing Company, Reading, Mass., 1972, p. 292). They can only be used to observe domain walls. Individual domains, which are magnetic regions in which the direction of magnetization is uniform, look similar using these methods. A major disadvantage of the Bitter method is the need of a lengthy specimen preparation, which involves mechanical polishing and electropolishing of the surface before application of a colloidal Fe.sub.3 O.sub.4 suspension (recipes for the preparation of a colloidal suspension of magnetite are given by Elmore, W. C., Phys. Rev. 54, 309 (1938) and Kittel, C. and Galt, J. K., Solid State Phys. 3, 439 (1956)). A limitation of Lorentz microscopy is the requirement that the sample must be very thin, approximately 1000 .ANG. or less, to transmit electrons. Additionally, an electron microscope equipped for Lorentz microscopy represents a very substantial capital investment.
The Kerr and Faraday Effects are common techniques used in the study of magnetic materials (Morrish, A. H., "The Physical Principles of Magnetism," Robert E. Krieger Publishing Company, Malabar, Fla., 1983, p. 374). They can distinguish one domain from another but supply no information about the domain walls. The Kerr Effect involves the rotation of the plane of polarization of a light beam during reflection from a magnetized sample. Since the amount of rotation is much less than one degree, the method is not easy to apply. In the Faraday Effect, the plane of polarization of a light beam is rotated as it is transmitted through the magnetized sample. This technique is therefore limited to very thin samples which can transmit light. In many materials including those in this disclosure, the Faraday Effect is not applicable because the sample does not transmit light. In both of these techniques (i.e., Kerr and Faraday Effects), the magnet interacts directly with the light beam to generate an observable effect.
While the above techniques are useful, none is a fast, inexpensive technique suitable for studying essential details of the process of magnetization reversal, i.e., change in direction of magnetization of domains, which is essential in characterizing magnetic materials in both bulk and thin film form.
Coercivity is a very important magnetic property of a permanent magnet. It describes the magnet's resistance to magnetization reversal, i.e., the change in direction of magnetization by 180.degree.. Coercivity is often a significant factor in determining the commercial value of a permanent magnet. In many cases coercivity of the material of which the magnet is constructed is controlled by impurities which are magnetically soft and not desired. Magnetization reversal occurs preferentially at these soft spots and subsequently may spread throughout the entire material. Thus the existence of these impurity soft spots is linked to the utility of the magnet. A rapid, inexpensive, non-destructive method for observing these soft spots would be very desirable. No existing techniques have all these attributes.
It is known that a liquid crystal (or a liquid crystal/pleochroic dye combination) can be oriented by a magnetic field. Priestley, E. B.; Wojtowicz, P. J.; Sheng, P. in "Introduction to Liquid Crystals"; Plenum Press, N.J., 1975, p. 115 report that about 1 oe of magnetic field is the equivalent of about 1 volt/cm of electric field in orienting liquid crystals. Further, it is known that orientation of liquid crystals can be detected optically. This has been shown by many people such as: Creagh, L. T.; Kmetz, A. R.; Reynolds, R. A., "Performance Characteristics of Nematic Liquid Crystal Display Devices," IEEE Trans. Electron Devices, p. 672 (1971) and Heilmeier, G. H.; Zanoni Applied Physics Letters, Vol. 13, No. 3, 1968, p. 91.
Permanent magnets are usually fabricated by batch processes. The yield is less than 100%, i.e., not all batches lead to a successful product. A rapid, inexpensive technique to assay a sample magnet from a batch would enable one during processing to make compositional adjustments, modify processing parameters, etc., and thus increase yield. The present invention is a method for rapid, inexpensive assaying of permanent magnet materials.
A method is provided to use (liquid crystals or liquid crystals/pleochroic dyes) and polarized light to detect and quantify the soft spots in the magnetic material as it is processed. To applicant's knowledge, there are no existing methods which use the interaction between polarized light and (liquid crystals or liquid crystals/pleochroic dyes) to study magnetic properties, i.e., coercivity, saturation magnetization and magnetization reversal of magnets.